Light emitting diodes (LEDs) are becoming increasingly widely used in automobile design because of their longer lives and lower cost compared to the incandescent bulbs they replace. Present day automotive designers are specifying LEDs not only for indicator lamps and alphanumeric digits but also for high power illumination lamps such as center high mounted stop lights. LED stop lights require very high brightness, but often only over a very limited viewing angle. FIG. 1 shows the current U.S. federal standard for LED center high mounted stop light brightness in candela as a function of viewing angle.
In order to be cost competitive with incandescent bulbs, an LED stop light must contain only a minimum number of individual LED lamps. The number of individual lamps can only be minimized if each lamp extracts substantially all of the light flux from the LED chip and concentrates the light within the useful viewing angle. Light flux outside of the viewing angle is wasted and might have been available to increase brightness within the viewing angle. Commercially available indicator lamps, which are designed according to the principles of imaging optics and standard manufacturing techniques, fail to concentrate sufficient light flux within the narrow required viewing angle. The imaging optics design constraint that the emitting surface is imaged onto the viewing plane makes design of a cost effective LED illumination lamp using imaging optics very difficult.
An alternative design approach known as non-imaging optics has been used successfully in the design of high efficiency solar collectors An additional degree of design freedom is available in non-imaging optics since there is no requirement that the emitting surface be imaged onto the viewing plane. An informative discussion of non-imaging optics may be found in the textbook "The Optics Of Nonimaging Concentrators" by W. T. Welford and R. Winston Specific examples of the use of non-imaging optics in solar collectors may be found in the U.S. patents (e.g., U.S. Pat. Nos. 3,923,381 and 3,957,031) issued to Dr. Roland Winston.
In accordance with the illustrated preferred embodiments of the present invention, the inventors have used the concepts of non-imaging optics to provide a high efficiency LED illumination lamp that is well adapted for use in an external automobile light such as a stop light. The lamp, which produces a very bright output over a preselected limited viewing angle, consists of two primary stages plus an optional lens stage. The first stage is a flux extractor which supports the LED and concentrates the three dimensional light flux into a desired angle, such as .+-.45.degree., relative to the optical axis. The second stage is a light pipe which continues the concentration to a final desired viewing angle. By using this second stage, instead of continuing the flux extractor's compound parabolic shape, the inventors have greatly simplified manufacturing and improved the cost and reliability of the lamp. One of a number of lenses may be used to increase the apparent illuminated area of the lamp or to allow a decrease in the physical height of the lamp.
In an alternative preferred embodiment of the present invention, a diffusant may be used to scatter the LED light. A bulk diffusant may be located within the structure of the lamp itself or a diffusing layer may be positioned over the lamp. The diffusant operates to increase the apparent size of the illuminated area, to decrease the brightness of the lamp and to increase the flux divergence to an approximately Lambertian distribution at the diffusant surface.